Study On Fatigue Performance And Span Of A7N01P-T4 Aluminum Alloy With Hole Defects

Macroscopic defects such as holes and grooves is one of the important reasons for material fracture.Materials with multiple defects often can not accurately evaluate the material life.In this paper,the fatigue crack initiation and propagation behavior of A7N01P-T4 aluminum alloy with multi-hole defects at room temperature in noncorrosive medium was studied by introducing drilling holes.In this paper,the fatigue initiation and propagation behavior of A7N01P-T4 aluminium alloy with multi-hole defects in room temperature non-corrosion medium was studied.The fatigue life and strength of the material with multi-hole defects were predicted by infrared thermography and FKM.High-cycle fatigue tests were carried out on the GPS100 digital high-frequency fatigue machine for multi-hole defect specimens with different spacing and different number of defects at stress ratio R=0.1.It was found that S1/D2=1.2 was the critical point of interaction enhancement.The influence of number and spacing on fatigue was characterized by area parameter and spacing influence coefficient.The crack initiation law and mechanism of multi-hole defect were studied by microscopy and scanning electron microscopy.It was found that A7N01PT4 multi-hole defect specimens had few undissolved coarse impurities and hard-brittle phases distributed at grain boundaries.The crack initiation was caused by the "invasion" and "extrusion" of slip zone on the surface of the specimens,which resulted in the formation of discontinuous interface and secondary stress concentration on the surface.When the spacing of defects decreases,the stress field around the side hole will gradually increase,and the stress field around the center hole will form a high stress area(relative to yield strength).The micro-cracks will initiate on the surface of the side hole in the form of increased probability.The results of crack propagation with different defect distribution show that the defect can passivate the crack tip and decrease the stress intensity factor when the crack propagates through the defect.Because the crack always propagates along the direction of minimum resistance,the stress field gradient around the defect promotes and guides the crack growth.Combining with the test results,the effect of defect on crack growth is mainly hindered.The effect of superposition of stress field between holes on crack rate is less than that of stress intensity factor when cracks encounter hole defects.The fatigue cracks of 7N01P-T4 aluminium alloy specimens with defects propagate in the way of intergranular and transgranular mixing,and the high stress field will provide the driving force for the intergranular propagation of cracks.The temperature evolution curve of A7N01P-T4 aluminum alloy with defects can be roughly divided into four stages during fatigue process: initial temperature rise stage,temperature stable slow drop stage,temperature stable slow rise stage and temperature rapid rise fracture stage.Under the action of alternating load,the temperature rise rate and temperature rise around the hole defect will be greater than that far away from the defect because of the heat generation effect of the grain friction and dislocation movement caused by the stress concentration around the defect.The fatigue life prediction of A7N01P-T4 aluminum alloy specimens with the defect was carried out by infrared thermal imager,and the prediction model of A7N01P-T4 aluminum alloy specimens with multiple defects was established at a certain frequency and stress ratio.The predicted results were consistent with the experimental results,which indicated the feasibility of using infrared thermal imager to predict the life of materials with multiple defects.On the premise of material testing,fatigue notch coefficient,average stress and notch roughness,the fatigue strength of A7N01P-T4 aluminum alloy with the defect is predicted.The predicted results are more accurate.